FR2573094A1 - MOLYBDENE ALLOY AND PROCESS FOR PRODUCING THE SAME - Google Patents

Abstract

THE INVENTION CONCERNS A MOLYBDENE ALLOY AND ITS MANUFACTURING PROCESS. THE MOLYBDENE ALLOY CONTAINS FROM 0.2 TO 1.0 BY WEIGHT OF AN OYXIDE OF A SPECIFIED METAL PREPARED BY ADDITION OF A SALT DUDIT METAL IN A DISSOLVED FORM WITH MOLYBDENE AND OR A MOLYBDENE OXIDE OR TO A MIXTURE OF DIFFERENT COMPONENTS BASED ON MOLYBDENE; THE MIXTURE IS THEN REDUCED TO A TEMPERATURE UP TO 1150C USING HYDROGEN, THE REDUCED METAL IS COMPRESSED WITHOUT THE ADDITION OF A PREFERRED BINDER UNDER A PRESSURE BETWEEN 150 TO 300MPA THEN THE PRODUCT IS SINTERED AT A COMPRESSED TEMPERATURE BETWEEN 1750 AND 2200C; PREFERENTIALLY ADDED ZIRCONIUM SALTS WITH MOLYBDENE OXIDE. APPLICATION TO FURNACES AND THE NUCLEAR INDUSTRY.

Description

The present invention relates to a molybdenum-based alloy which is

creep resistant to

  high temperatures and which is stable even under an atmosphere of

  reducing gas, and also its manufacturing process.

  The high melting point of molybdenum, its good

  modulus of elasticity, its stability at relative temperatures

  high corrosion resistance makes it possible and necessary for many

  varied and extremely important applications.

  To further improve the properties of molybdenum, especially its resistance to high temperatures, molybdenum alloys containing carbides have been used. The alloy known as TZC, which contains 1.25% Ti, 0.15 to 0.5% Zr and 0.15 to 0.3% C, is particularly resistant to high temperatures. TZM alloy, which contains less carbon and 0.4 to 0.5% Ti, 0.06 to 0.12% Zr and 0.01 to 0.04% C, does not have the strength, however. at the high temperatures of TZC alloy. These alloys are used as cores for injection molding of aluminum and for dies during die casting of brass and stainless steel. The use of these alloys is also recommended for the manufacture of gas turbine components, valves and nozzles in rocket propulsion units. A disadvantage of said alloys however consists in the fact that they are of a more difficult and more expensive manufacture than that of molybdenum alone, because they must be sintered under a partial vacuum in the presence

  dry hydrogen to prevent compositional changes.

  tions and that they acquire only the properties

  desired after a subsequent heat treatment.

  In addition to its use in the metals industry and rocket technology, molybdenum has become increasingly important as a corrosion-resistant material for the construction of chemical equipment.

  and as a building material in the furnace industry.

  Molybdenum is of particular importance as a material for forming heat conductive elements and radiation shields in high temperature furnaces; because it can be used in particular in a hydrogen atmosphere up to 2000C and in vacuum up to 1600'C. AT

  such high temperatures, the alloys mentioned

  above have improved properties compared to molybdenum alone. Nevertheless, the service life and the creep resistance of the abovementioned alloys and other proposed alloys are still limited and therefore require

an upgrade.

  For example the alloy HT - molybdenum containing

  potassium and silicon - recrystallises and loses its resistance

  tance. This alloy can only be processed if strict manufacturing conditions are observed, which must absolutely be followed. Attempts have also been made to use composite alloys which contain vanadium, boron and carbon in addition to molybdenum (DE-OS 32 23 618) or molybdenum plus titanium, zirconium, hafnium, or a rare earth metal, in any case at the same time as carbon (German Offenlegungsschrift 26 17 204), for producing alloys of high heat stability and high strength

  to chemicals, without loss of hardness.

  None of the above-mentioned alloys, especially alloys containing carbide, are stable in a humid hydrogen atmosphere, for example in furnaces using refractory bricks and flame curtains, because under such conditions a decarburization occurs. , which causes a corresponding loss of

creep resistance.

  It is therefore necessary to have a molybdenum alloy which is also resistant to such

extreme conditions.

  We have developed a molybdenum alloy that has improved properties over molybdenum alone and with respect to the alloys mentioned above, while not requiring such treatment conditions.

rigorous or demanding.

  The present invention relates to a method of manufacturing a molybdenum alloy which comprises the steps of mixing molybdenum and / or molybdenum oxide with a solution of a salt of a suitable metal (as defined herein) to be subjected to mixing after, if necessary, drying, grinding and screening, at a reducing hydrogen atmosphere at a temperature of not more than 1150 ° C, to perform another process to form a shaped part by known processing techniques molybdenum powder, and sintering under an atmosphere of hydrogen at a temperature of between 1750 ° C and 2200 ° C, without addition of carbon or a compound

  containing carbon at any stage of the process.

  It has surprisingly been found that molybdenum alloys containing only small amounts of specified metal oxide and no percentage of carbon are stable even at temperatures above 1600 ° C, for example also in a moist hyrogen atmosphere, and maintain their creep resistance and their

  mechanical resistance in such extreme conditions.

  The metal oxide content of an alloy prepared by a process according to the present invention is defined by the metal salt used which is converted

  in oxide during the different stages of implementation.

  The mixing step may be carried out either by the conventional technique of mixing the molybdenum component with the metal salt solution to form a slurry or by spraying the solution onto the molybdenum-based component to obtain a solution. wet mix; this last technique is the one that we have developed and which surprisingly gives good

  results and major advantages over the conventional

  than. When direct mixing occurs, the slurry formed must be dried, milled and filtered to obtain the powder needed for another process. We have found that by spraying the solution onto the molybdenum-based component, the formed wet mix could be used without any further treatment being required for a solution.

  other conditioning for the formation of the alloy.

  As a result, preferably the metal salt solution is sprayed onto the

  molybdenum in a suitable mixer, for example a mixture of

  mechanical rotary shaft. The mixer used must obviously be resistant to corrosion, for example by being provided with

  a rubber coating. The solution can be sprayed

  by using a suitable sprayer or a suitable nozzle apparatus. It has been found particularly advantageous to use a compressed air sprayer. Preferably, when all the material to be sprayed has been used, the

  component corresponding to molybdenum and the corresponding component

  The metal salt layers are further mixed in the mixer for a short period of time, preferably a half hour, and especially a period of minutes. This additional mixture ensures a homogeneous distribution of the components. The mixture obtained is usually

  wet but not wet and it is not necessary to

  kill a drying, grinding or filtering treatment

at this stage.

  The molybdenum component may be pure molybdenum or molybdenum oxide. Alternatively, a mixture of molybdenum and one or more oxides may be used

  molybdenum, or a mixture of molybdenum oxides.

  Raw materials for the manufacture of such alloys include industrially pure molybdenum dioxide or molybdenum trioxide, such as those obtained by treating the ore through ammonium molybdate or molybdic acid. As mentioned above, it is also possible to use a mixture of the aforementioned oxides or a mixture of said oxides with pure molybdenum. Molybdenum oxide is intimately mixed with a solution of a suitable metal salt in a suitable apparatus. The solution is preferably an aqueous solution and is suitably acidified, if necessary, to maintain the salt in solution. Depending on the oxide of alkene used, the oxide mixture is reduced to one or two stages of metal in a hydrogen atmosphere at temperatures up to 1150 ° C, preferably at a temperature within the range.

  between 850 and 11500 C, and in particular at 1050 ° C.

  During this process, the metal salts added in small amounts undergo hydrolysis to form the hydroxides and they finally remain homogeneously distributed in the

metal in the form of oxides.

  By the term "suitable metal" used herein, it is meant a metal which, in the form of a salt,

  is a solution with a suitable solvent, this solution

  producing an oxide of the metal under the operating conditions

  according to the process according to the invention, without further decomposition or permanent flooding from the solid phase. The term "metal" refers herein to a metalolide or an element having metal-like properties. Thus metals whose oxides melt at temperatures of 1500 ° C. or more and which form a dispersed oxide in a hardened molybdenum alloy are suitable and the oxides mentioned above are, for example, those which remain solid and in dispersion in molybdenum under the conditions of the process, for example during sintering, or which melt during sintering and which then precipitate to form a phase

  dispersed in molybdenum metal during cooling.

  Metals whose oxides melt during the reduction or sintering processes and which do not precipitate for

  forming an oxide dispersed in the cured material does not

should not.

  Particularly suitable metals include aluminum, barium, calcium, cerium, chromium, afnium, magnesium, silicon, strontium,

  thorium, yttrium, zirconium and similar metals.

  In the process according to the invention, it is preferable to use a salt of aluminum, chromium, afnium, titanium or zirconium. It is particularly preferred to use a salt of aluminum, chromium or zirconium when the alloy

  resulting must be used in the nuclear industry.

  It is possible to use in the process according to the invention a mixture of salts which produce a

  alloy containing the corresponding mixture of metal oxides

  c. It is also possible to use a salt of two

appropriate metals.

  The salt used should be a salt which is soluble, or which can be kept in solution, in a suitable solvent and which produces an oxide of the appropriate metal under the operating conditions corresponding to the process according to the invention. The salt is preferably a chloride, a

iodide, sulphate or nitrate.

  In particular, the salt of a suitable metal used is zirconium nitrate. Advantageously this salt

  is used in an aqueous solution that can be fresh-

  prepared or that may be an available solution

  commercially, for example a 20% solution by weight.

  The aqueous solution of zirconium nitrate can be acidified

  for its application in the process according to the invention.

tion.

  The amount of salt used is advantageously chosen so as to obtain an alloy containing from 0.2 to 1% by weight of an oxide of a suitable metal, preferably from 0.25 to 0.6% by weight of a metal oxide and in particular

  from 0.3 to 0.6% by weight of said oxide.

  The solvent used in the process according to the invention may be any suitable solvent, and preferably

some water.

  When using the metal alloy in nuclear technology, however, it must be ensured that only metal salts, whose oxides have a thermal neutron capture section as small as possible, are added to the base component. of molybdenum. Since said alloys do not contain carbon, no other reducing agent that would reduce the metal oxides is present after the reduction of the oxide of

molybdenum metal.

  It is also usually unnecessary to use

  binders at any stage of the process according to the invention.

  tion. As a result, no impurities, in particular no carbon, enter the alloy and its properties

  therefore are not affected.

  The powder obtained in the process according to the invention can be treated according to techniques

  molybdenum powder packaging, without the use of

  conditions or special precautions. For example, the use of a partial vacuum, a dry hydrogen atmosphere in the sintering step, is unnecessary, although an atmosphere of hydrogen at atmospheric pressure or a low

  pressure is preferably employed.

  The resulting metal powder is suitably filtered initially through a 1.65 mm mesh screen and then through a 0.06 mm mesh screen. The grains thus obtained have a particle size of between 2 and 8 microns (average particle size obtained with a Fisher fine sieve). The powder is compressed at a pressure of between 150 and 300 mPa to form profiled parts, for example rods or bars, and is heated in a retort furnace for a period of between 3 and 72 hours at a temperature of between 1750 and 2200 ° C, preferably about 1850 ° C, depending on the size of the part and the particle size of the powder. During this process, the metal is sintered to form a solid block and then has a density greater than 91% of the theoretical density. If this density is not reached, it is necessary that the sintering be continued for several hours. When forming bars, it is possible to laminate them in the form of sheets in a known manner by making

  intervene the temperatures and the corresponding reduction steps

  spawning with pure molybdenum. The surface is normally shiny; it can be pickled with acids to obtain a finish

mast.

  Another advantage of this alloy is that it can be welded by conventional argon arc welding processes using, if necessary, filler wires (or electrodes) which are made of the same material to be used. molybdenum base or a different material. Electron beam welding of metal is also possible. The metal is advantageously annealed at 950 C after welding. The metal can be used in a wide variety of applications where it is important to achieve high temperature resistance and resistance

  corrosion, especially under reducing conditions.

  The following example illustrates the invention.

Example

  kg of pure molybdenum dioxide are introduced

  in a cement mixer coated with rubber or corrosion resistant. 0.69 kg of zirconium nitrate are dissolved in 4 liters of water with the addition of a small amount of nitric acid. Nitric acid is intended to prevent premature hydrolysis of zirconium nitrate and to maintain the latter in solution. The zirconium nitrate solution is sprayed by means of a suitable nozzle onto the molybdenum oxide which is kept moving in the mixer. When the addition of the nitrate solution is complete, mixing is continued for another period of 5 to 15 minutes. The mixture is then made wet but not wet. The mixture is then heated, without intermediate drying, in molybdenum alloy or nickel alloy trays and in a hydrogen atmosphere, the temperature rising slowly to 1150 C. While the molybdenum oxide is reduced in metal, zirconium nitrate decomposes to form zirconium dioxide and is uniformly distributed in the molybdenum powder. The resulting metal powder is filtered, first through a sieve of about 1.7 mm mesh aperture and then through a sieve of about 0.064 mm mesh aperture. Grains with a particle size greater than 1.7 mm and grains with a particle size greater than 0.064 mm are rejected. Fine grains with a particle size less than 0.064 mm are compressed to

  form rectangular bars without the addition of a binder.

  The bars are heated for 45 hours in a retort furnace at 850 C in a hydrogen atmosphere. After the sintering process, the material has a density corresponding to approximately 93% of the theoretical density. The metal can be made into sheets

by rolling.

  Analogous alloys are obtained using

  aluminum, chromium, titanium or hafnium salts.

Claims (14)

  A method of making a molybdenum alloy which comprises the steps of mixing molybdenum and / or molybdenum oxide with a solution of a salt of a suitable metal (as defined herein) to subject the mixture after if necessary, drying, grinding and screening, at a reducing hydrogen atmosphere at a temperature of not higher than 1150 ° C, to perform another implementation to form a profiled part by known techniques for treating molybdenum powder , and sintering under a hydrogen atmosphere at a temperature of between 1750 ° C and 2200 ° C, without addition of carbon or   of a carbon-containing compound at any stage of the process.   2. Method according to claim 1, characterized in that the solution of a metal salt is sprayed on the molybdenum and / or the molybdenum oxide and is mixed   in a mixer suitable for producing a wet mixture.   3. Method according to one of claims 1 or 2,   characterized in that molybdenum dioxide, or molybdenum trioxide, or a mixture of said substances is used.   4. Method according to any of the claims   1 to 3, characterized in that the amount of metal salt used is such that the prepared alloy contains between 0.2 and 1.0% by weight of an oxide of the metal in the alloy prepared result.   5. Method according to claim 4, characterized in that the amount of metal salt used is such that the alloy prepared contains between 0.25 and 0.6% by weight an oxide of the metal.   6. Process according to any one of the claims   1 to 5, characterized in that the salt used is a salt of a metal chosen from aluminum, barium, calcium, cerium, chromium, hafnium, magnesium, silicon, strontium, thorium, yttrium, and zirconium, or a mixture of two or more of said salts is used. 7. Process according to Claim 6, characterized in that a salt of aluminum, chromium, hafnium, titanium or zirconium is used, or a mixture of two or more than two of said salts.   8. Method according to claim 7, characterized   in that a zirconium salt is used.   9. Process according to any one of the claims   6 to 8, characterized in that the salt is a chloride, a iodide, sulphate or nitrate.   10. Process according to claim 9, characterized   in that the salt is zirconium nitrate.   11. Process according to any one of the claims   1 to 10, characterized in that the other treatment is performed in the absence of a binder.   12. Process according to any one of the claims   1 to 11, characterized in that the sintering is carried out under a hydrogen atmosphere at atmospheric pressure or at a reduced pressure.   13. Molybdenum alloy prepared by a process   according to any one of claims 1 to 12.   The molybdenum alloy according to claim 13, which contains molybdenum and between 0.2 and 1% by weight of a   oxide of a suitable metal (as defined herein).